Rotating polygon scanning mirrors are typically used in laser printers to provide a “raster” scan of the image of a laser light source across a moving photosensitive medium, such as a rotating drum. Such a system requires that the rotation of the photosensitive drum and the rotating polygon mirror be synchronized so that the beam of light (laser beam) sweeps or scans across the rotating drum in one direction as a facet of the polygon mirror rotates past the laser beam. The next facet of the rotating polygon mirror generates a similar scan or sweep, which also traverses the rotating photosensitive drum but provides an image line that is spaced or displaced from the previous image line. Since increasing the rotational speed of a polygon mirror is accompanied by even greater increases in cost, printer manufacturers using polygon mirrors as a drive engine often use two or four equally spaced and parallel lasers all directed toward and reflected from a single facet of the polygon mirror.
There have also been prior art efforts to use a less expensive flat mirror with a single reflective surface to provide a scanning beam. For example, a dual axis or single axis scanning mirror may be used to generate the beam sweep or scan instead of a rotating polygon mirror. The rotating photosensitive drum and the scanning mirror are synchronized as the drum rotates in a forward direction to produce a printed image line on the medium that is parallel with the beam scan or sweep generated by the pivoting mirror and orthogonal to the movement of the photosensitive medium.
However, with the single axis mirrors, the return sweep will traverse a trajectory on the moving photosensitive drum that is at an angle with the printed image line resulting from the previous or forward sweep. Consequently, use of a single axis resonant mirror, according to the prior art, required that the modulation of the reflected light beam be interrupted as the mirror made the return sweep or completed its cycle, and was then turned on again as the beam started scanning in the original direction. Using only one of the sweep directions of the mirror, of course, reduces the print speed. Therefore, to effectively use a scanning mirror to provide bi-directional printing, the prior art attempts of bi-directional scanning required that the sweep plane also be controlled in a direction perpendicular to the scan such that the sweep of the mirror in each direction generates images on a moving or rotating photosensitive drum that are always parallel. To achieve such parallel lines required continuous perpendicular adjustment of the sweeping beam, and such adjustment can be accomplished by the use of a dual axis torsional mirror or a pair of single axis torsional mirrors. It has been discovered, however, at very high print speeds both forward and reverse sweeps of a single axis mirror may be used, and that no orthogonal adjustment is necessary.
Texas Instruments presently manufactures torsional dual axis and single axis pivoting MEMS mirrors fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 115–125 microns. The dual axis layout may, for example, consist of the mirror surface being supported on a gimbal frame by two silicon torsional hinges, whereas for a single axis device the mirror is supported directly by a pair of torsional hinges. The mirror may be of any desired shape, although an elliptical shape having a long axis of about 4.0 millimeters and a short axis of about 1.5 millimeters is particularly useful. Such an elongated ellipse-shaped mirror is matched to the shape at which the angle of a light beam is received. The gimbal frame used by the dual axis mirror is attached to a support frame by another set of torsional hinges.
Texas Instruments also manufactures a single axis mirror comprising two layers of silicon and a magnet. The multilayered mirror allows the selecting of the most effective torsional hinge while at the same time reduces the flexibility or bending of the mirror surface at high oscillation speeds.
These mirrors manufactured by Texas Instruments are particularly suitable for use as the scanning engine for high-speed laser printers and visual display.
According to the prior art, torsional hinge mirrors were initially driven directly by magnetic coils interacting with small magnets mounted on the pivoting mirror at a location orthogonal to and away from the pivoting axis to oscillate the mirror or create the sweeping movement of the beam. In a similar manner, orthogonal movement of the beam sweep was also controlled by magnetic coils interacting with magnets mounted on the gimbals frame at a location orthogonal to the axis used to pivot the gimbals frame. Inexpensive and dependable magnetic drives can also be used and set up in such a way to maintain the pivoting device at its resonant frequency. Further, although the reflecting surface of a scanning mirror can be of almost any shape, including square, round, elliptical, etc., an elongated elliptical shape has been found to be particularly suitable.